The administration of supplemental oxygen is well established form of therapy for patients with chronic respiratory difficulties. For example, oxygen is administered not only to those patients with chronic obstructive lung disease who are hypoxemic, but also to patients with pulmonary insufficiency caused by such disorders as interstitial lung disease, sleep apnea, and kyphyoscoliosis. Conventionally, the administration of prescribed supplemental oxygen has been on a continuous basis, the flow of oxygen not being interrupted, and the medical profession has adapted a general technique for prescribing such therapy typically by specifying the gas application in terms of continuous flow, for example 3 liters per minute or the like.
Continuous, long-term oxygen therapy is expensive, a substantial portion of the gas being wasted. In this regard, approximately one-third of the respiratory cycle is spent in inhalation and two-thirds in exhalation. Thus, a device that delivers oxygen during inspiration only while assuring adequate alveolar gas exchange will serve to conserve the gas, while prolonging the duration of a given supply. Those supplies, of course, may be derived from generally immobile oxygen concentrators, liquid oxygen systems, or large oxygen cylinders. Lowering the gas demands for a given patient, in turn, evokes savings in the transportation and operation of such supplies. Recognizing the inefficiencies of continuous oxygen therapy, investigators have looked to respiratory-phased or responsive systems wherein oxygen is, in effect, pulsed to the patient under a control logic based upon the physician prescribed gas flow rate and the breathing characteristics. One such breathing characteristic adduced in the course of investigation has been that the initial 60% of any inspiration time is devoted to the introduction of "fresh" gas into the alveoli. Gas introduced for the remaining 40% of a given inspiration essentially has no oxygen transfer effect and represents what has been referred to as "dead-space" ventilation. See generally:
"Efficacy of Pulsed Oxygen Delivery During Exercise" by McDonnell, Wanger, Senn and Cherniack, Respiratory Care, vol. 31, No. 10, Oct. 1986 PA0 "Oxygen Conserving Devices" by O'Donahue, Jr., Respiratory Care, vol. 32, No. 1, Jan., 1987. PA0 "Efficacy of the Oxymizer Pendant in Reducing Oxygen Requirements of Hypoxemic Patients, by Gonzales, Huntington, Romo, and Light, Respiratory Care, vol. 31, No. 8, Aug., 1986 PA0 "Oxygen Transport and Utilization", by Dantzker, Respiratory Care, vol. 33, No. 10, October, 1988 PA0 "Pulmonary Responses to Exercise", by Lough, Respiratory Care, vol. 34, No. 6, June, 1989. PA0 "The Future of Home Oxygen Therapy", by O'Donohue, Jr., Respiratory Care, vol. 33, No. 12, December, 1988
Any approach to controlled oxygen administration also should account for the lifestyle and emotion driven characteristics of the patient. Typically, this patient initially will be prone to panic mentally in consequence of an apprehension of perceived suffocation. The normally-encountered response is a rapid breathing rate, for example about 20 breaths per minute, and a breathing technique which is described as "shallow", typically exhibiting a rapid or gasping commencement of inhalation. Notwithstanding the mentally induced difficulties leading to strained breathing patterns, breathing becomes problematic to the patient when transitioning from a rest on an ambulatory or moving activity, inasmuch as the body imposes a heightened oxygen demand which was not present in a rest condition. As a consequence of this increased demand resulting from even minor movement, patients have a natural tendency not to carry out even the most menial or basic of activities to exacerbate the requirements for their care. One approach of therapists has been to train the patient to breathe more slowly and deeply to improve oxygen uptake efficacy. For example, patients undergoing pulmonary rehabilitation may be taught a breathing routing while walking wherein inspiration time extends over an interval of two steps and expiration time extends for three steps. During this exhalation time the lips are pursed to form a positive pressure. Therapists have been observed to instruct the patient to "smell the roses" in describing this form of training exercise. Such categorizations as well as others are employed to cause the patient to remember to breathe in a trained manner. When the patient forgets, the apprehension again sets in with attendant anxiety and return to fast paced shallow breathing.
In view of the foregoing, it is desirable that any respiration phased or pulsed oxygen delivery system aid in the very procedure of training the patient to breathe under optimal self-control. The gas delivery approach, while achieving a desired gas concentration, should also provide a basic physician prescribed oxygen flow rate, e.g. in liters per minute, but also accommodate unschedulated ambulatory activity with temporary, higher gas transfer rates, and without resort to manual mechanical valve changes and the like. Where such patient activity increases with attendant increased oxygen demand, the pulse based delivery systems should retain a capability for temporarily exceeding prescribed oxygen delivery rates during patient activity excursions. Following such excursions and the development of patient rest-based breathing stability, the gas delivery rate should return to physician prescribed levels without resort to manual procedures. Throughout all such gas exchange alterations, the patient should receive perceptible cuing that all is in order and requisite oxygen is available. With such cuing, patient apprehension which otherwise may be experienced, is more readily dispelled to the overall comfort and confidence of that individual.
The implementation of this desirable respiratory phase oxygen administration has posed a variety of technical problems to equipment designers. For example, systems carrying out pulse oxygen delivery based upon detecting negative inhalation pressure occurring during a breathing cycle should employ highly sensitive pressure sensing devices. Typically, these sensors utilize a diaphragm-actuated wheatstone bridge configuration which provides an output transitioning from a balance to an unbalance condition in response to patient inspiration. Over periods of time and with temperature transitions, these devices tend to drift or become unbalanced, leading to erratic or insensitive performance. Where the sensors are used in geographic regions of high altitude, their sensitivity may be severely impaired. Thus, some form of response adjustment which is simple for the user is desired.
Where a desired high sensitivity to negative inhalation pressure is achieved, erratic pulse system behavior also is experienced due to pneumatic transients which are developed within gas delivery conduits as an oxygen supply pulse interval terminates. Often, the system will react to these transients by generating uncontrolled, noise-induced pulses, a phenomenon sometimes referred to as "autopulsing".
Because the pulsing valve systems generally are actuated by the energization of current-demanding windings of solenoid valves, any desired utilization of rechargeable batteries with the system is rendered difficult. Often, even though recharging networks are supplied, these networks are incapable of bringing rechargeable battery sources back to optimum voltage levels. The system also should incorporate techniques for diverting from a controlled pulse actuated supply of medical gas to a continuous flow. Such switch-over to a different gas flow path may be occasioned, for example, with loss of power or for applications, for example, requiring the nebulized applications of medicaments. While valving for carrying out such procedures is readily available, a small, inexpensive and practical form of flow metering for such alternative utilization of the application systems has been elusive to designers. Finally, the devices employed for oxygen application generally have been restricted to the outputs of only certain forms of gas pressure regulators. It will be desirable to provide control devices which may perform with more than one regulated gas supply input.